1. Field of the Invention
The present invention relates to a solid state laser oscillator, and particularly to a solid state laser oscillator capable of highly efficient operation.
2. Description of the Related Art
Solid state laser devices that employ semiconductor laser diodes (LD's) as pumping light sources and utilize solid state laser media, in which rare earth ions (or transition metal ions) are doped in a host, are being actively developed. Many of these solid state laser devices employ solid state laser crystals (solid state laser media) in which neodymium (Nd) is doped as active ions in host crystals of yttrium aluminum garnet (Y3Al5O12), yttrium vanadate (YVO4) and the like, or glass. In these cases, the absorption coefficient α with respect to pumping light is comparatively high (5-30 cm−1), and almost all of the optical power of the pumping light from the LD's can be absorbed with a crystal length d (medium length) of several millimeters. Here, the absorptance within a single pass in a laser medium is expressed by the following formula (1).ηabs=1−exp(−αd)  (1)
For example, Nd:YVO4 (doping concentration: 1 at %) can obtain a pumping light absorption coefficient α=cm−1 (pumping wavelength: 808.9 nm), and a single pass absorptance ηabs can reach 95% with a crystal length of 1 mm. That is, pumping power can be efficiently absorbed with a crystal length of 1 mm. Nd:YAG (doping concentration: 1 at %) can obtain a pumping light absorption coefficient α=5 cm−1 (pumping wavelength: 808 nm), and a single pass absorptance ηabs of 63% can be obtained with a crystal length of approximately 2 mm. Generally, increasing the absorptance of pumping light is necessary to improve the total efficiency (efficiency from electricity input to light output) of solid state laser devices. In addition, because pumping light can be absorbed with a short crystal length, miniature LD pumped solid state laser devices are capable of being realized.
In contrast, for solid state laser media having low absorption coefficients, there are cases in which the single pass absorptance is conspicuously low, on the order of 1% to 20%, if the crystal length is only several millimeters. For example, in the case that hot band pumping (pumping wavelength: 885 nm) is to be performed on Nd:YAG, the absorption coefficient α=1.6 cm−1, and only about 15% of the pumping light is absorbed with a crystal length of 1 mm. Further, Ti:Sapphire crystals (sapphire solid state laser medium doped with titanium ions) exhibit an absorption coefficient α=1.6 cm−1 (pumping wavelength: 532 nm), and the absorptance is extremely low, at ηabs=11% with a crystal length of 2 mm, and ηabs=16% even with a crystal length of 3 mm. Note that it is possible to achieve a single pass absorptance ηabs>90% even with media having low absorption coefficients α, by increasing the crystal length d such that the product ad of the absorption coefficient and the crystal length is greater than or equal to 2.3. However, this results in a drawback that the crystal becomes large, therefore increasing the cost of the device.
Even in cases that the absorption coefficient is comparatively high, in the case that quasi three level laser oscillation is to be performed, it is preferable to avoid increases in the crystal length d. This is because reabsorption of the emitted laser light occurs, which leads to a higher oscillating threshold. For example, Nd:YAG oscillation at 946 nm (4F3/2 to 4I9/2), and Yb:YAG (2F5/2 to 2F7/2) are quasi three level systems, and it is difficult to realize high absorption of pumping light and a low oscillating threshold simultaneously. There are laser crystals other than Nd:YAG that exhibit quasi three level laser oscillation. Emission of visible light, such as blue oscillation (480 nm, 3P0 to 3H4) of Pr:YLF (LiYF4), blue oscillation (488 nm, 3P0 to 3H4) of Pr:YAG, blue oscillation (482 nm, 1G4 to 3H6) of Tm:YLF, and green oscillation (551 nm, 4S3/2 to 4I15/2) of Er:YLF all correspond to quasi three level laser oscillation.
Various techniques for improving the absorptance of pumping light have been proposed. The simplest and most often used technique is that in which a mirror for reflecting pumping light is provided at an end facet of or outside a crystal, thereby causing a double pass therethrough (refer to “Diode-pumped tunable Yb:YAG miniature lasers at room temperature: modeling and experiment”, T. Taira et al., IEEE J. Selected Topics on Quantum Electronics, Vol. 3, Issue 1, pp. 100-104, 1997). In this case, the absorption length becomes twice the crystal length, and the absorptance increases. However, in the case that the absorption coefficient is extremely low, a mere doubling of the absorption length has limited effects. For example, in the aforementioned hot band pumping of Nd:YAG, if the medium length L is increased from 1 mm to 2 mm, ηabs=27%, which is still extremely low, even though it has been approximately doubled. Causing multiple passes greater than 2 is difficult with a simple optical system.
Meanwhile, a technique has been realized, in which image relay is employed while shifting the axis of pumping light, to cause multiple passes to occur. In actuality, an optical system that causes pumping light to pass through a thin discoid Yb:YAG crystal (thickness of 0.2-0.5 mm) sixteen times has been proposed (“A 1-kW CW thin disc laser”, C. Stewen et al., IEEE J. Selected Topics on Quantum Electronics, Vol. 6, Issue 4, pp. 650-657, 2000). In this case, however, a complex optical system must be provided with precise positioning.
The examples described above are cases in which pumping light is treated as incoherent light. On the other hand, there are cases in which the coherency of pumping laser beams is utilized. An example of such a configuration employs a narrow line width semiconductor laser that oscillates at a single frequency (single longitudinal mode), and utilizes resonator effects provided by an external resonator. In this case, the oscillating frequency of the pumping laser is tuned with the longitudinal mode of the external resonator by temperature or current control, to maintain the resonating state of the pumping laser within the external resonator. A laser crystal is provided within the external resonator, and the pumping light resonates a great number of times within the external resonator. Thereby, even if the single pass absorptance of the laser crystal is low, the same effects as a tenfold to a hundredfold increase in the absorption length can be obtained, and pumping light can be efficiently absorbed (“Pump-resonant excitation of the 946-nm Nd:YAG laser”, J. P. Cuthbertson et al., Optics Letters, Vol. 16, Issue 6, pp. 396-398, 1991; “Efficient diode-laser-pumped 946 nm Nd:YAG laser withresonator-enhanced pump absorption”, W. J. Kozlovsky et al., IEEE J. Quantum Electronics, Vol. 28, Issue 4, pp. 1139-1141, 1992; and U.S. Pat. No. 5,048,047).
The example disclosed by Cuthbertson et al. employs a single frequency oscillating semiconductor laser (wavelength: 810 nm) as a pumping light source 101, and pumps an Nd:YAG crystal 102, as illustrated in FIG. 14. The rear side 102a (toward the semiconductor laser) of the Nd:YAG crystal 102 and an output mirror 103 function as a resonator for an emitted laser light beam 108 as well as for a pumping light beam 107. The rear side 102a of the Nd:YAG crystal is formed to be highly reflective with respect to the emitted laser light beam 108, and partially reflective (reflectance: 85%) with respect to the 810 nm pumping light beam 107. The output mirror 103 is formed to be partially reflective (reflectance: 99.5%) with respect to the emitted laser light beam 108, and highly reflective with respect to the pumping light beam 107. The resonator length is 10 mm. The thickness of the Nd:YAG crystal is 0.3 mm, and the Nd doping concentration is 1 at %. An output light beam 109 (wavelength: 946 nm) having an output power of 1 mW is obtained with a pumping power of 10 mW. Although there is no disclosure regarding single pass absorption, it can be estimated to be 15%. In this example, oscillation at 946 nm is of the quasi three level type. If the single pass absorption is higher, loss due to reabsorption at a lower laser level increases, which results in decreased oscillation efficiency. Therefore, the extremely thin laser crystal is employed in this configuration.
Kozlovsky et al. and U.S. Pat. No. 5,048,047 also disclose laser devices that realize oscillation at 946 nm employing Nd:YAG crystals. These laser devices comprise a laser crystal 105 with a thickness of 0.33 mm, as illustrated in FIG. 15. The end facets of the laser crystal 105 are formed as resonator mirrors that construct a resonator for both a pumping light beam 107 and an emitted laser light beam 108. Although not shown in the figure, stable operation is realized by providing a wavelength tuning circuit. In this example, an output light beam 109 having a wavelength of 946 nm and an output power of 30 mW is obtained with a pumping power of 600 mW. Single pass absorption is 18%.
In the conventional laser devices illustrated in FIG. 14 and FIG. 15, single frequency (single longitudinal mode) continuous wave laser diodes are utilized as pumping light sources. However, it is difficult to increase the output of continuous wave pumping lasers. In order to emit single frequency laser beams, it is necessary to provide etalons, diffraction gratings, or distribution feedback (DFB) structures within resonators, to cause oscillation at only a desired laser oscillation frequency. This causes loss with respect to other frequencies, and often results in decreased pumping laser beam output. This necessarily results in decreased output of a solid state laser device that uses the pumping laser. Generally, the output of single frequency oscillating semiconductor lasers is limited to be within a range of approximately 10 mW to 100 mW.
As described above, there are two conventional techniques which have been proposed in order to increase the effective absorptance of solid state laser media having low single pass absorptance: (1) using a multiple pass pumping optical system; and (2) using a narrow line width single frequency pumping laser and an external resonator to increase absorption.
However, technique (1) has a problem that the pumping optical system becomes extremely complex or large, and technique (2) has a problem that the narrow bandwidth of the pumping light beam reduces the pumping power, which precludes high power laser output. To date, a miniature and high output solid state laser device has not been realized.